Apparatus and a method for characterizing multiphase effluents

ABSTRACT

An apparatus for characterizing an effluent formed by a multiphase fluid mixture is described, said apparatus comprising a source emitting gamma rays at a plurality of energy levels through said effluent towards a detector block that takes account of the photons it receives as a consequence thereof to deduce therefrom the attenuation of said rays by said effluent. The detector block includes a filter for selectively detecting the photons it receives at a first energy level and at a second energy level, said first and second levels being predetermined amongst the energy levels of radiation from the source.

The invention relates to apparatus and to a method for characterizingmultiphase effluents. A particular application of the invention lies intaking measurements concerning the composition of effluents coming fromoil wells, which effluents are constituted by multiphase fluidstypically comprising three phases: two liquid phases, namely crude oiland water, and a gas phase which is based on hydrocarbons.

In the oil industry, the traditional practice for characterizing thecomposition of multiphase effluents consists in separating the effluentinto its component phases and in measuring the phases as separated inthis way. However, that technique requires separators to be installed onsite, even though separators are pieces of equipment that are expensiveand voluminous, and when testing wells that method also requiresadditional pipes.

Numerous proposals have been put forward to develop techniques making itpossible to avoid making use of such separators. A description of suchdevelopments can be found in the publication SPE 28515 (SPE AnnualTechnical Conference, New Orleans, Sep. 25–28, 1994) by J. Williamsentitled “Status of multiphase flow measurement research”.

French patent FR 2 764 065 describes a method of characterizing an oilwell effluent directly inside the production tubing without it beingnecessary to perform separation. In that method, a gadolinium 153 sourceis used to emit gamma rays into the effluent at a first energy level ofabout 100 kilo electron-volts (keV), and also at a second energy levelof about 40 keV, and the attenuation of the gamma rays at each of thosetwo levels is measured after the rays have passed through the effluent.The measurements taken make it possible to determine the oil/water/gasfractions in the effluent. Combining those measurements withmeasurements from a flowmeter makes it possible to deduce production foreach of the three phases.

However, since the lifetime (period) of gadolinium 153 is short (lessthan one year) it is of interest for taking measurements over a shortduration such as periodic measurements in oil wells, but it isunsuitable for taking measurements on a permanent or semipermanentbasis. When measurements are taken by tools that are fixed innon-removable manner in or on a well, it is necessary to use a sourcewhose lifetime is long enough to ensure that the tool continues tooperate throughout the lifetime of the well, without it being necessaryto change the source.

Barium 133 has already been used as another source for emitting gammarays and it has the advantage of a lifetime that is longer. That sourcetherefore makes it possible to implement permanent measurements. Inconventional manner, such measurements are performed by measuring theattenuation of gamma rays at a first energy level of 32 keV and at asecond energy level of 356 keV. In such applications, the energy of thenuclear instrumentation of the tool is stabilized on the peak at 356keV, i.e. the peak in the photons received by the detector block at thecorresponding energy level is calibrated so as to enable said photons tobe counted accurately. That gives rise to various drawbacks. Firstly,the energy peak corresponding to radiation at 356 keV does not have theappearance of a Gauss curve. The distribution of the curve relative toits high point is very broad and highly asymmetrical due to the presenceof other energy peaks (276 keV, 303 keV, and 384 keV) that are not fullyresolved by the scintillator crystal of a detector. That gives rise tothe major drawback of making it difficult to stabilize the detector, sostabilization is only approximate. Secondly, the photons emitted at 356keV tend to deposit only a fraction of their energy in the detectorcrystal, thus disturbing measurements at lower energy.

An object of the invention is thus to remedy those drawbacks byproposing apparatus and a method for characterizing oil well effluent insuch a manner that the fractions and the densities of the phases in saideffluent are determined in particularly reliable manner.

To this end, the invention provides apparatus for characterizing aneffluent formed by a multiphase fluid mixture, said apparatus comprisinga source emitting gamma rays at a plurality of energy levels throughsaid effluent towards a detector block that takes account of the photonsit receives as a consequence thereof to deduce therefrom the attenuationof said rays by said effluent. According to the invention, the detectorblock includes a filter for selectively detecting the photons itreceives at a first energy level and at a second energy level, saidfirst and second levels being predetermined amongst the energy levels ofradiation from the source.

This solution is particularly advantages since the apparatus makes itpossible to decrease the number of photons that are uselessly detectedby the detector block, thereby significantly increasing the efficiencyof the measurements. This decrease makes it possible to minimizecorrections for pile-ups, i.e. the addition effects between two photonswhich are detected simultaneously at different energy levels so thatthey are taken to be constituted by a single photon at a higher energylevel. This solution also makes it possible to decrease the dead timethat corresponds to the time during which the detector block isunavailable between detecting the arrival of one photon and actuallyallocating a certain energy level thereto. These two improvementsincrease the accuracy with which attenuation measurements are performedfor given source activity or they make it possible to increase sourceactivity so as to reduce statistical error. Finally, by not respondingto a large fraction of the photons that are emitted at other energylevels, the apparatus of the invention makes it possible to decreasesignificantly the measurement errors encountered with apparatuses knownin the prior art.

In a preferred embodiment of the invention, the filter comprises ascintillator crystal whose dimensions are such that said crystal mainlydetects photons that are emitted at said first and second energy levels.

This embodiment is advantageous since it makes it possible in a mannerthat is very simple and low in cost to implement the function ofdiscriminating between different photon energy levels in the apparatusof the invention. The larger the dimensions of the scintillator crystal,the more said crystal can detect high energy photons, so a consequentialadaptation of said dimensions thus makes it possible quite simply tocount only a fraction of said photons received by the detector block.With the apparatus of the invention, it is possible to detect mainlythose photons which are emitted at the lowest energy levels whiledetecting only a smaller fraction of the photons received at higherenergy levels.

In an advantageous embodiment of the invention, the gamma ray emittingsource comprises barium 133.

This solution is advantageous since by using a source having a lifetimethat is long it enables the apparatus to be used for permanentlymeasuring the composition of an effluent, e.g. down an oil well, on aninhabited platform, or at the bottom of the sea.

In a preferred embodiment of the invention, the first energy level issituated substantially at 32 keV, and the second energy level issituated substantially at 80 keV. In this preferred example, theapparatus of the invention also includes a stabilization loop locked onthe first and/or the second energy level.

These two energy levels are preferred because photon counting turns outto be more accurate at these levels. The decrease in the quantity ofhigh energy photons taken into account reduces the risks of pile-ups andreduces dead times. In addition, since the stabilization loop is lockedon one of these two energy levels, or on both of them, the fact thatthey are close to each other makes it possible to minimize errors inallocating a photon to one or other of the levels during counting.Finally, the energy peak corresponding to radiation at 80 keV has theappearance of a Gaussian curve with distribution relative to its highpoint that is narrow, thereby guaranteeing accurate calibration in termsof energy received and photon allocation to a particular spectrum line.The same applies to the peak at 32 keV.

The invention also provides a method of characterizing an effluentformed by a multiphase fluid mixture, in which method:

-   -   gamma rays are emitted into said effluent by means of a gamma        ray emitting source at a plurality of energy levels;    -   the photons received from said source after passing through said        effluent are filtered by means of a detector block so as to        detect selectively the photons which correspond to gamma rays at        a first energy level and at a second energy level, said first        and second energy levels being predetermined amongst the energy        levels at which the source radiates; and    -   the attenuation of the gamma rays at said first and second        energy levels is measured on the basis of the count rate        corresponding to the number of photons detected after filtering        so as to deduce therefrom the fractions of the various phases in        said effluent.

Other advantages and characteristics of the invention become clear fromthe following description given by way of example and made withreference to the accompanying drawing, in which:

FIG. 1 is a diagrammatic view in section and in perspective of a pieceof apparatus in accordance with the invention; and

FIG. 2 shows two photon detection spectra, one of which is obtained bymeans of apparatus of the invention.

FIG. 1 shows a section of pipe 1 in which a multiphase fluid mixtureflows. By way of example, this pipe is situated in an oil well and themultiphase fluid comprises water, oil, and gas. Apparatus 2 of theinvention is installed on said pipe. The apparatus comprises a source 3and a detector 4 placed on opposite sides of the pipe 1, the pipe beingprovided with “windows” of material that is a poor absorber of photonsat the energies under consideration. In the embodiment shown, the source3 comprises barium 133 which produces gamma rays at various energylevels: 30 keV, 80 keV, 276 keV, 303 keV, 356 keV, and 384 keV. However,any other chemical or electronic gamma ray source could be used.

In the advantageous embodiment of the apparatus of the invention, asshown in FIG. 1, the section of pipe 1 includes a converging Venturi.The source 3 and the detector 4 are situated on either side of thethroat of the Venturi, i.e. the narrowest portion thereof. This enablesmeasurements to the composition of the effluent to be coupled withmeasurements of its flow rates, e.g. using the method described inFrench patent No. FR 97/10648.

The detector 4 comprises a scintillator crystal 40, such as an NaIcrystal, together with a photomultiplier 41. In the detectors known inthe prior art, the photomultiplier 41 takes account, amongst otherthings, of photons corresponding to two energy levels referred to as a“high” energy level and as a “low” energy level, correspondingsubstantially to 32 keV and to 356 keV, respectively. These energylevels are such that the “high energy” count rate is responsiveessentially to the density of the fluid mixture while the “low energy”count rate is also sensitive to the composition of the liquid mixture,thus making it possible to determine the water content of the effluent.However, the scintillator crystal is such that all of the photonsemitted by the source 3 are detected and taken into consideration,thereby giving rise to errors in measuring the energy levels which areparticularly intended for measuring the composition of the effluent.

The scintillator crystal 40 of the invention is such that, on thecontrary, only a fraction of the photons emitted by the source 3 aredetected. The length d of the crystal 40 is previously determined sothat this crystal mainly detects photons having an energy level that issubstantially less than or equal to 80 keV, the major fraction ofphotons at higher energy levels passing through the crystal withoutbeing detected. As a result, the photomultiplier 41 takes account onlyof “low” energy level photons corresponding substantially to 30 keV andof “high” energy level photons corresponding substantially to 80 keV.

This selective detection is particularly advantageous because itdecreases the total number of photons that are detected and thereforedecreases the errors that are statistically inherent to such counting.The apparatus of the invention preferentially detects useful photons(those at low energy), thereby reducing errors, in particular those dueto pile-ups and also reducing the dead time of the detector block. Thus,errors are no longer induced by the sheer number of photons to bedetected since overall this number is reduced.

For characterizing oil effluent, the “high” energy level presents aremarkable property whereby the gamma ray attenuation coefficient perunit mass is substantially the same at this level for fresh water, saltwater, and oil. As a result, the “high energy” attenuation makes itpossible to determine the density of the mixture without it beingnecessary for this purpose to perform any auxiliary measurements(attenuation coefficients and densities) in order to determine theindividual phases of the effluent.

At this energy level, the attenuation A as measured by the detector 21can be expressed by the following relationship:A=D _(v)·ν_(m)·ρ_(m)  [1]where D_(v) is the distance traveled through the fluid, i.e., in thiscase, the diameter of the section of pipe 1, where ν_(m) is theattenuation coefficient per unit mass, and ρ_(m) is the density of themultiphase mixture.

Since the attenuation coefficients per unit mass of water and oil havevalues that are substantially identical at the above-mentioned energylevel, and since the contribution of gas is negligible because of itsvery low density, the attenuation coefficient per unit mass ν_(m), andthus the product D_(v)·ν_(m) that appears in equation [1] can beconsidered as being substantially constant, and independent of thedensities of the oil and water phases. Under such conditions, the “highenergy” attenuation A_(he) is a highly advantageous indicator of thedensity ρ_(m) of the mixture.

The photomultiplier 41 of the detector block 4 of the invention also hasa stabilization loop set to the “high” energy level at 80 keV. Thisstabilization loop serves to enable the photomultiplier to allocate anenergy level to a photon detected by the scintillator crystal so as toenable the photon to be counted appropriately. This setting of thephotomultiplier on the 80 keV peak generates the calibration enablingother photons to be allocated to the other energy levels that correspondto them. With apparatus of the invention, the stabilization loop isparticularly effective, for several reasons.

Firstly, as can also be seen in FIG. 2, the peak corresponding to the 80keV level is in the form of a very narrow Gaussian curve. It is thuseasier to stabilize apparatus on this narrow detection range rather thanon a broader range as is necessary with the 356 keV peak which has theform of a much wider Gaussian curve. Thereafter, since the “high” energylevel and the “low” energy level are closer together, it is also easy toguarantee minimum deviation of the stabilization for the “low” energylevel.

In another embodiment of the apparatus of the invention, a secondstabilization loop can advantageously be included based on the “lowenergy” peak, thereby further increasing accuracy of counting. The countrates at the two energy levels in question thus turn out to be veryreliable.

To illustrate the advantages of the apparatus of the invention, FIG. 2shows two spectra corresponding to the count rates of two detectorblocks. Both spectra are normalized on the 80 keV peak for comparisonpurposes. The first curve A corresponds to the spectrum obtained with adevice whose stabilization loop is locked on the 80 keV energy level andfor which the length d of the scintillator crystal is large, being about1 inch (25.4 mm). The second curve B, corresponds to the spectraobtained from apparatus of the invention in which the stabilization loopis indexed on the 80 keV energy level and for which the length d of thescintillator crystal is short: about half an inch (12.7 mm). Oncomparing these two curves, it can clearly be seen that the amplitude ofthe peak corresponding to the 356 keV level is much lower in curve Bthan in curve A, simply because the apparatus of the invention detectsvery few photons at this energy level. The shaded portion corresponds tothe increase in count rate obtained by the detector block of curve Bcompared with the count rate obtained by the detector block of curve A.It can be seen that curve B is low at energies above 100 keV whereas theheight of curve A at such energy levels is higher. The greater heightbeneath curve A comes from the way in which the measurements are so tospeak “polluted” by the known Compton phenomenon effected by the 356 keVphotons (a fraction of their energy is detected while the remainingfraction is detected as being at a lower level and therefore appears inthe graph). As a result, curve B presents a spectrum whose appearance ismore representative of the photons that are genuinely useful at the 80keV and 30 keV levels.

With the apparatus of the invention, the method of determining themultiphase composition of an effluent turns out to be particularlysimple and reliable. This composition is determined using the followingprinciples: if the attenuation of the gamma rays induced by each of thecomponents taken separately at 80 keV and at 30 keV and if the densityof each of these components are all known, then the attenuation of gammarays at 80 keV characterizes the density and thus the gas fraction inthe multiphase effluent and combining data at 80 keV and at 30 keV forsaid mixture characterizes the water fraction in the effluent.

This calculation can be presented in matrix form as follows:

$\begin{bmatrix}A^{H} \\A^{L} \\1\end{bmatrix} = {\begin{bmatrix}A_{o}^{H} & A_{w}^{H} & A_{g}^{H} \\A_{o}^{L} & A_{w}^{L} & A_{g}^{L} \\1 & 1 & 1\end{bmatrix} = \begin{bmatrix}\alpha_{o} \\\alpha_{w} \\\alpha_{g}\end{bmatrix}}$where the oil, water, and gas fractions are the unknowns α_(i), where Hrepresent the “high” energy level, L represents the “low” energy level,o represents the oil phase, w represents the water phase, and grepresents the gas phase.

To determine the attenuations of the various components and of themixture at 30 keV and at 80 keV, the method of the invention thusconsists in using the source 3 to emit gamma rays through the section ofpipe 1 towards the detector block 4. Thereafter, the scintillatorcrystal 40 of the detector block detects photons that correspond mainlyto the 30 keV and 80 keV energy levels after they have passed throughthe effluent, with the length d of said crystal being insufficient toretain all of the high energy photons. Comparing the count ratesobtained in this way with those obtained when the section of pipe isempty makes it possible to deduce the attenuation that is due to themultiphase mixture, and finally to deduce the composition of saidmixture.

The method and apparatus of the invention thus makes it possible toobtain photon count rates that are particularly reliable so as toprovide the composition of a multiphase fluid mixture. When the sourcewhich emits the gamma rays at a plurality of energy levels is barium133, then the apparatus of the invention can be used in applications forpermanently measuring the composition of an effluent, e.g. flowing inthe downhole pipes of an oil well or in a pipe situated on the seabed orat the surface.

1. Apparatus for characterizing an effluent formed by a multiphase fluidmixture, comprising: a source emitting gamma rays at a plurality ofenergy levels through the effluent so as to be attenuated thereby; and adetector block comprising a photomultiplier, wherein the detector block:is positioned so as to receive photons from the source that have passedthrough the effluent; includes a filter for simultaneously capturingphotons at first and second energy levels, the first and second energylevels photons passing through the filter and said first and secondenergy levels being selected from the plurality of energy levels ofgamma rays emitted from the source; deduces the attenuation of the gammarays by the effluent by taking into account the received photons at thefirst and second energy levels; and ensures that the photomultipliertakes account only of the first and second energy levels.
 2. Apparatusas claimed in claim 1, wherein the filter comprises a scintillatorcrystal whose dimensions are such that the crystal mainly detectsphotons that are emitted at the first and second energy levels. 3.Apparatus as claimed in claim 2, wherein the scintillator crystal has alength selected to detect photons having energy levels not greater thana higher of the first and second energy levels.
 4. Apparatus as claimedin claim 3, wherein the scintillator crystal has a length selected sothat a major fraction of photons having energy levels substantiallyhigher than the higher of the first and second energy levels are notdetected.
 5. Apparatus as claimed in claim 2, wherein the scintillatorcrystal has a length of not more than half an inch.
 6. Apparatus asclaimed in claim 1, wherein the gamma ray emitting source comprisesbarium
 133. 7. Apparatus as claimed in claim 1, wherein the first energylevel is situated substantially at 30 keV and the second energy level issituated substantially at 80 keV.
 8. Apparatus as claimed in claim 1,further comprising a stabilization loop locked on the first energy leveland/or a second stabilization loop locked on the second energy level. 9.Apparatus as claimed in claim 1, further comprising a stabilization looplocked on the second energy level.
 10. Apparatus as claimed in claim 1,further comprising a first stabilization loop locked on the firm energylevel and a second stabilization loop locked on the second energy level.11. Apparatus as claimed in claim 1, further comprising a convergingVenturi coupled to the source and the detector.
 12. Apparatus as claimedin claim 1, wherein the multiphase mixture comprises oil, water, andgas.
 13. Apparatus as claimed in claim 1, wherein the detector blockprovides first and second count rates corresponding to a number ofphotons detected at the first and second energy levels respectively. 14.A system for use in measuring flows from wells comprising: a pipecarrying an effluent flow from a well and adapted to receive a sourceand a detector block; a source, adapted to be coupled to the pipe,emitting gamma rays at a plurality of energy levels trough the affluentso as to be attenuated thereby; and a detector block comprising aphotomultiplier, wherein the detector block: is positioned so as toreceive photons from the source that have passed through the effluent;includes a filter, adapted to be coupled to the pipe, for simultaneouslycapturing photons at first and second energy levels, the first andsecond energy levels photons passing through the filter and said firstand second energy levels being selected from the plurality of energylevels of gamma rays emitted from the source; deduces the attenuation ofthe gamma rays by the effluent by taking into account the receivedphotons at the first and second energy levels; and ensures that thephotomultiplier takes account only of the first and second energylevels.
 15. A system as claimed in claim 14, wherein the pipe comprisesa Venturi.
 16. A system as claimed in claim 15, wherein the pipe isadapted to receive the source and detector block at the throat of theVenturi.
 17. A system as claimed in claim 15, further comprising aplurality of pipes, each connected to a well so as to carry the effluenttherefrom.
 18. A system as claimed in claim 15, wherein the effluentflow comprises a mixture of oil, water and gas.
 19. A system as claimedin claim 15, wherein the filter comprises a scintillator crystal whosedimensions are such that the crystal mainly detects photons that areemitted at the first and second energy levels.
 20. A system as claimedin claim 19, wherein the scintillator crystal has a length selected todetect photons having energy levels at least equal to a higher of thefirst and second energy levels.
 21. A system as claimed in claim 20,wherein the scintillator crystal has a length selected so that a majorfraction of photons having energy levels substantially higher than thehigher of the first and second energy levels are not detected.
 22. Asystem as claimed in any of claims 19, wherein the scintillator crystalhas a length of not more than half an inch.
 23. A system as claimed inclaim 15, wherein the gamma ray emitting source comprises barium 133.24. A system as claimed in claim 15, wherein the first energy level issimulated substantially at 30 keV and the second energy level issituated substantially at 80 keV.
 25. A system as claimed in claim 15,further comprising a stabilization loop locked on the first energy leveland/or a second stabilization loop locked on the second energy level.26. A system as claimed in claim 15, further comprising a stabilizationloop locked on the second energy level.
 27. A system as claimed in claim15, further comprising a first stabilization loop locked on the firstenergy level and a second stabilization loop locked on the second energylevel.
 28. A system as claimed in claim 15, wherein the detector blockprovides first and second count rates corresponding to a number ofphotons detected at the first and second energy levels respectively. 29.A method of characterizing an effluent formed by a multiphase fluidmixture, comprising: emitting gamma rays into the effluent by means of agamma ray emitting source at a plurality of energy levels; filteringphotons resulting from emission of the gamma rays through the effluentby means of a detector block so as to simultaneously retain photons itreceives both at a first energy level and at a second energy level, thefirs and second energy levels being selected from the plurality ofenergy levels of the source; the filtering step being made such that thefirst and second energy levels photons pass through the filtering;detecting only the first and second enemy levels by means of aphotomultiplier; and measuring attenuation of the gamma rays at thefirst and second energy levels on the basis of count rates correspondingto the number of photons detected after filtering so as to deducetherefrom fractions of various phases in the effluent.
 30. A method asclaimed in claim 29, wherein the filtering is performed to detectphotons having energy levels not greater than a higher of the first andsecond energy levels.
 31. A method as claimed in claim 29, wherein thefiltering is performed so that a major fraction of photons having energylevels substantially higher than a higher of the first and second energylevels are not detected.
 32. A method as claimed in claim 29, whereinthe first energy level is situated substantially at 30 keV and thesecond energy level is situated substantially at 80 keV.
 33. A method asclaimed in claim 29 wherein the multiphase mixture comprises oil, water,and gas.
 34. A method as claimed in claim 29, further comprisingdetermining a density of the multiphase mixture.